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• https://en.wikipedia.org/wiki/Carboniferous#Rocks_and_coal

• https://arstechnica.com/science/2016/01/why-was-most-of-the-earths-coal-made-all-at-once/

• The question wants us to discuss as to why most of the world’s coal was formed during a particular period/ era- Carboniferous period.

• Write a few introductory lines about the importance of coal and introduce the Carboniferous period.

• In the main body, discuss why most of the coal was formed during the Carboniferous period. E.g

• • While coal deposits formed both before and after the Carboniferous, this period provided the mother lode. It occurred a bit over 300 million years ago.

• • The evolution of the wood fiber lignin and the bark-sealing, waxy substance suberin variously opposed decay organisms so effectively that dead materials accumulated long enough to fossilise on a large scale.

• • The second factor was the lower sea levels that occurred during the Carboniferous as compared to the preceding Devonian period. This promoted the development of extensive lowland swamps and forests in North America and Europe.

• • During this period club mosses grow to the size of trees while insects also reached comparatively gigantic proportions due to the higher-than-modern oxygen concentration.

• • The reason all that oxygen was present is the vast burial of organic material before it could be eaten by oxygen-respiring organisms.

• • And while oxygen rose, atmospheric CO2 fell, eventually leading to glacial conditions. It was a massive carbon-cycle experiment that mirrored our current one but with carbon moving in the opposite direction, from the atmosphere into the ground, where it formed the coal.

• • During the Carboniferous, the Pangaea supercontinent was coming together. And in a tropical swath along the equator, a mountain range (now the Appalachians) was being pushed up by continental collision.

• • On either side of that growing mountain range, the crust bowed downward a bit as a result. Those ever-deepening bedrock buckets were positioned right beneath soggy tropical wetland regions etc.

• Conclude with a brief discussion on the Indian coal.

Answer:

The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago (Mya), to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means “coal-bearing”. The Carboniferous coal beds provided much of the fuel for power generation during the Industrial Revolution and are still of great economic importance.

Most of the coal was formed during the Carboniferous period. The reasons for the same are:

• Abundance of Lignin: The appearance of wood tissue and bark-bearing trees. The evolution of the wood fibre lignin and the bark-sealing, waxy substance suberin variously opposed decay organisms so effectively that dead materials accumulated long enough to fossilise on a large scale. Most organisms couldn’t eat the tough barks, except for the white-rot fungus that lives on dead trees.
• Reduced Sea Level: The second factor was the lower sea levels that occurred during the Carboniferous as compared to the preceding Devonian period. This promoted the development of extensive lowland swamps and forests in North America and Europe.
• Biotic Evolution: Coal deposits formed both before and after the Carboniferous, this period provided the mother lode. It occurred a bit over 300 million years ago and species of club mosses grow to the size of trees. Insects also reached comparatively gigantic proportions due to the higher-than-modern oxygen concentration.
• Abundance of Oxygen: The reason all that oxygen was present is the vast burial of organic material before it could be eaten by oxygen-respiring organisms. Atmospheric CO2 fell, eventually leading to glacial conditions. It was a massive carbon-cycle experiment with carbon moving in the opposite direction, from the atmosphere into the ground, where it formed the coal.
• Late Evolution of Bacteria: Based on a genetic analysis of mushroom fungi, it was proposed that large quantities of wood were buried during this period because animals and decomposing bacteria had not yet evolved enzymes that could effectively digest the resistant phenolic lignin polymers and waxy suberin polymers. They suggest that fungi that could break those substances down effectively only became dominant towards the end of the period, making subsequent coal formation much rarer.

However, there are objections to the above theory as lignin isn’t even the only type of organic matter in Carboniferous-age coals. Thus, all the Carboniferous world’s lignin couldn’t have made its way into coal and at least some of it must have decayed.

Alternative Theory:  

The formation of coal requires two steps.

• First, there is a need of swampy environment where peat can accumulate in low-oxygen conditions that wards off decay.
• Second, the need to bury the whole organic matter quite deeply, allowing pressure and temperature to turn the peat into coal.

Explanation: During the Carboniferous, the Pangaea supercontinent was coming together. And in a tropical swath along the equator, a mountain range (now the Appalachians) was being pushed up by continental collision. On either side of that growing mountain range, the crust bowed downward a bit as a result. Those ever-deepening bedrock buckets were positioned right beneath soggy tropical wetland regions. The end result was lots of deeply buried peat.

There was one other time in North American history with significant coal formation, and that was a period bracketing the mass extinction of the dinosaurs. Then, too, there was mountain-building (this time the Rockies) with a neighbouring basin, and hot, wet climate conditions.

In India, the Gondwana coal (about 250 mya) belongs to the carboniferous period. Gondwana coal makes up to 98 per cent of the total reserves and 99 per cent of the production of coal in India.